Although aluminum is the third most abundant element in the earth's crust, it is never found in nature as an element, but rather occurs in more than 250 different minerals. The most important mineral groups are the alumino-silicates, clays (which are weathered silicates), and the hydrated oxides, such as bauxite. The industrial production of aluminum is based on extraction from an oxide-containing mineral, namely, bauxite, which is today's dominating raw material for the beneficiation of alumina used in the Hall-Heroult electrolytic reduction of the oxide to pure aluminum.
Usually containing between about 10-30 wt % iron (III) oxide, 4-8 wt % silica, and 2-5 wt % titania as major impurities, crude bauxite is dried, ground, and reacted in the well-known Bayer process with soda ash and lime in steel digesters to dissolve the alumina and to allow its separation from the remaining red mud. The purified alumina is transferred to an electrochemical cell where, under the Hall-Heroult process, elemental aluminum metal is formed in an electrolytic process using an Na.sub.3 AlF.sub.6 --AlF.sub.3 --Al.sub.2 O.sub.3 electrolyte (i.e., a cryolite-aluminum fluoride-alumina mixture). The reduction of alumina in the cell occurs at about 970.degree. C. according to the following simplified reaction: EQU Al.sub.2 O.sub.3 +3/2 C=2 Al(liq)+3/2 CO.sub.2 (1 atm)
Because of the huge power requirements for the direct electrochemical production of aluminum metal from alumina in the Hall-Heroult process, extensive research has been conducted to find alternative processes. Decades of research have failed to achieve an economically viable process for the direct carbothermic reduction of alumina, because the process is extremely complex, with numerous side reactions occurring. These side reactions are further enhanced and augmented by the introduction of silicon, iron, and other impurities often found in the crude ore reactants, such as bauxite. Even if pure alumina is used, the best result of the direct carbothermic reduction of alumina is the preparation of a mixture of aluminum metal and aluminum carbide, with the resulting need to decompose the carbide to the metal at about 2100.degree. C.
Failing to find an economical process for the direct carbothermic reduction of alumina, the industry has directed its attention to the two-step process of converting alumina to aluminum chloride and then further reducing the aluminum chloride to aluminum metal. In the Toth process, for example, AlCl.sub.3 is made from alumina, and pure manganese is then used for the reduction of aluminum chloride to elemental aluminum. Because the Toth process uses pure manganese, the process has been industrially and economically unattractive.
In 1973, Alcoa announced the development of a process for the production of aluminum by the electrolysis of aluminum chloride that is dissolved in a melt of alkali and alkaline earth chlorides. This electrolysis overcame the economic inviability of the manganese reduction step of the Toth process and culminated fifteen years of intensive Alcoa research. Armed with an economical method to reduce aluminum chloride to aluminum metal at significant energy savings, researchers turned to the production of aluminum chloride from alumina. Research continues today in an attempt to better understand the effects of such factors as the type of reductant, the type of aluminum-bearing material, the nature of the chlorinating agent, and the temperature and pressure effects upon the kinetics of the reaction for the different reactants and reaction conditions. Many patents have been issued relating to the production of AlCl.sub.3.
The carbothermic chlorination of alumina can be represented by the following simplified general reaction: EQU Al.sub.2 O.sub.3 +(n)C+3Cl.sub.2 =2AlCl.sub.3 +(2n-3)CO+(3-n)CO.sub.2 where 3.gtoreq.n.gtoreq.1.5.
As the reaction proceeds with the production of carbon dioxide and carbon monoxide, the carbothermic chlorination of alumina can be described by the following consecutive reactions: EQU Al.sub.2 O.sub.3 +3CO+3Cl.sub.2 =2AlCl.sub.3 +3CO.sub.2 EQU C+CO.sub.2 =2CO: Boudouard reaction
Current research can be categorized in two primary classifications, namely, research into alternative sources of aluminum (e.g., kaolinic clays) and research into the reaction mechanism and preparation of AlCl.sub.3 from alumina.
U.S. Pat. Nos. 4,105,752; 4,284,607; and 4,289,735 are representative of the research currently being done on the preparation of aluminum chloride. Of note, U.S. Pat. No. 4,284,607 (incorporated by reference) discusses the catalyzed chlorination of aluminous material to form aluminum chloride in the presence of alkali metal compounds as catalysts. Preferably, the catalyst is an alkali aluminum halide, such as potassium aluminum chloride, sodium aluminum chloride, rubidium aluminum chloride, and lithium aluminum chloride. The catalyst may be formed in situ by adding alkali metal compounds, such as potassium carbonate, potassium nitrate, or the like, to the reactants.
U.S. Pat. Nos. 4,139,602; 4,220,629; 4,083,927; 4,082,833; 4,288,414; 4,213,943; and 4,159,310 are representative of the current research on alternative sources of aluminum. These patents principally deal with the use of kaolinic clay as a raw material for the extraction of alumina and elemental aluminum from the ore. For example, U.S. Pat. No. 4,139,602 discloses a process for the preferential chlorination of alumina over silica in the carbo-chlorination of kaolinitic ores. The process uses alkali metal compounds with oxyanions to catalyze the reaction. These compounds are generally selected from the group consisting of alkali metal carbonates, sulfates, hydroxides, and oxides, preferably of sodium, potassium, and lithium. Additional oxyanions that may be used include thiosulfates, pyrosulfates, sulfites, nitrates, nitrites, oxalates, borates, bicarbonates, phosphates, and the like.
U.S. Pat. Nos. 4,220,629 and 4,083,927 disclose the preferential chlorination of alumina in kaolinic clays by use of a boron chloride catalyst to convert silicon chloride to aluminum chloride and silicon oxide. Similarly, U.S. Pat. No. 4,082,833 discloses the use of elemental sulfur or functionally equivalent sulfur-containing compounds as catalysts during the pre-halogenation steps of chlorinating kaolinic clays.
U.S. Pat. No. 4,213,943 discloses a staged reaction for the catalytic production of aluminum chloride from clay using an alkali metal compound and silicon tetrachloride to shift the chlorination reaction to aluminum chloride preferentially over silicon chloride. The catalyst preferably is an alkali metal compound and is normally an alkali aluminum halide of the type described in U.S. Pat. No. 4,284,607.
U.S. Pat. No. 4,283,371 discloses a continuous process for recovering substantially pure aluminum chloride from chlorination products of aluminum ore, and involves the separation of aluminum chloride from ferric chloride.
Extensive research has been undertaken to understand the catalysis of the Boudouard reaction. Research in this vein is illustrated by two articles to Dr. Y. K. Rao, et al., namely, On the Mechanism of Catalysis of the Boudouard Reaction by Alkali-Metal Compounds, 20 Carbon, No. 3, 207-212 (1982), and A Study of the Rates of Catalyzed Boudouard Reaction, 16 Carbon 175-184 (1978). These articles disclose that lithium oxide, lithium carbonates, and alkali carbonates are useful catalysts of the Boudouard reaction.
Although extensive research has been directed to the problem of preparing aluminum chloride economically, a viable solution has yet to be achieved.
Improvements in the catalyzed production of aluminum chloride are presented in this invention, which describes an economical process for producing AlCl.sub.3.